Dual cavity pressure structures

ABSTRACT

An apparatus includes a cavity within a substrate. A MEMS structure is within the cavity, wherein the cavity includes the MEMS structure. A trench is connected to the cavity, wherein the trench is not directly opposite the MEMS structure. An oxide layer lines the trench and the cavity. A seal layer seals the trench and traps a predetermined pressure within the cavity and the trench.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is divisional of prior application Ser. No. 15/071,499,filed Mar. 16, 2016, which claims the benefit of U.S. Provisional PatentApplication Ser. No. 62/134,455 filed Mar. 17, 2015, entitled “DUALCAVITY PRESSURE SCHEMES”.

BACKGROUND

MEMS (“micro-electro-mechanical systems”) are a class of devices thatare fabricated using semiconductor-like processes and exhibit mechanicalcharacteristics. For example MEMS devices may include the ability tomove or deform. In many cases, but not always, MEMS interact withelectrical signals. A MEMS device may refer to a semiconductor devicethat is implemented as a micro-electro-mechanical system. A MEMS deviceincludes mechanical elements and may optionally include electronics(e.g. electronics for sensing). MEMS devices include but are not limitedto, for example, gyroscopes, accelerometers, magnetometers, pressuresensors, etc. During fabrication, it may be desirable to create variousdifferent MEMS devices on the same wafer. Furthermore, it may bedesirable to form the various different MEMS devices with differentinternal pressures.

SUMMARY

An apparatus includes a cavity within a substrate. A MEMS structure iswithin the cavity, wherein the cavity includes the MEMS structure. Atrench is connected to the cavity, wherein the trench is not directlyopposite the MEMS structure. An oxide layer lines the trench and thecavity. A seal layer seals the trench and traps a predetermined pressurewithin the cavity and the trench.

These and other features and aspects of the concepts described hereinmay be better understood with reference to the following drawings,description, and appended claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a first substrate including a trench according to oneaspect of the present embodiments.

FIG. 2 shows the substrate with the addition of cavities according toone aspect of the present embodiments.

FIG. 3 shows a thermal oxide growth on the trench and the cavitiesaccording to one aspect of the present embodiments.

FIG. 4 shows the first substrate fusion bonded to a second substrateincluding standoffs according to one aspect of the present embodiments.

FIG. 5 shows the formation of MEMS features and eutectic bonding of thesecond substrate to a third substrate according to one aspect of thepresent embodiments.

FIG. 6 shows removal of a portion of the first substrate to reveal thetrench and adjust the pressure in one cavity according to one aspect ofthe present embodiments.

FIG. 7 shows the addition of a seal to set the adjusted pressure of thetrench cavity according to one aspect of the present embodiments.

FIG. 8 shows a first substrate including a trench according to oneaspect of the present embodiments.

FIG. 9 shows the substrate with the addition of cavities according toone aspect of the present embodiments.

FIG. 10 shows a thermal oxide growth on the trench and the cavitiesaccording to one aspect of the present embodiments.

FIG. 11 shows the first substrate fusion bonded to a second substrateincluding standoffs according to one aspect of the present embodiments.

FIG. 12 shows the formation of MEMS features and eutectic bonding of thesecond substrate to a third substrate according to one aspect of thepresent embodiments.

FIG. 13 shows removal of a portion of the first substrate to reveal thetrench according to one aspect of the present embodiments.

FIG. 14 shows removal of the exposed portion of the lining to reveal thetrench and adjust the pressure in one cavity according to one aspect ofthe present embodiments.

FIG. 15 shows the addition of seals to set the adjusted pressure of thetrench cavity according to one aspect of the present embodiments.

FIG. 16 shows an exemplary flow diagram for setting a pressure within atrench and cavity according to one aspect of the present embodiments.

FIG. 17 shows an exemplary flow diagram for sealing a first pressure ina second cavity and sealing a second pressure in a first cavityaccording to one aspect of the present embodiments.

DESCRIPTION

Before various embodiments are described in greater detail, it should beunderstood by persons having ordinary skill in the art that theembodiments are not limiting, as elements in such embodiments may vary.It should likewise be understood that a particular embodiment describedand/or illustrated herein has elements which may be readily separatedfrom the particular embodiment and optionally combined with any ofseveral other embodiments or substituted for elements in any of severalother embodiments described herein.

It should also be understood by persons having ordinary skill in the artthat the terminology used herein is for the purpose of describing thecertain concepts, and the terminology is not intended to be limiting.Unless indicated otherwise, ordinal numbers (e.g., first, second, third,etc.) are used to distinguish or identify different elements or steps ina group of elements or steps, and do not supply a serial or numericallimitation on the elements or steps of the embodiments thereof. Forexample, “first,” “second,” and “third” elements or steps need notnecessarily appear in that order, and the embodiments thereof need notnecessarily be limited to three elements or steps. It should also beunderstood that, unless indicated otherwise, any labels such as “left,”“right,” “front,” “back,” “top,” “middle,” “bottom,” “forward,”“reverse,” “clockwise,” “counter clockwise,” “up,” “down,” or othersimilar terms such as “upper,” “lower,” “above,” “below,” “vertical,”“horizontal,” “proximal,” “distal,” and the like are used forconvenience and are not intended to imply, for example, any particularfixed location, orientation, or direction. Instead, such labels are usedto reflect, for example, relative location, orientation, or directions.It should also be understood that the singular forms of “a,” “an,” and“the” include plural references unless the context clearly dictatesotherwise.

Unless defined otherwise, all technical and scientific terms used hereinhave the same meaning as commonly understood by persons of ordinaryskill in the art to which the embodiments pertain.

MEMS devices may include mechanical elements that work best when sealedin specific predetermined pressures. For example, a gyroscope MEMSdevice optimally operates at a lower pressure than an accelerometer MEMSdevice, which optimally operates at a higher pressure. Furthermore, someMEMS devices operate within different environments. For example, someMEMS devices may need a helium environment, while other MEMS devices mayneed a hydrogen environment. During fabrication, it may be desirable tocreate various different MEMS devices with different pressures and/orenvironments on the same wafer. Embodiments disclose methods forfabricating MEMS devices with different pressures and/or environments onthe same wafer, as well as the novel characteristics of the MEMS devicesfabricated by the disclosed embodiments.

Referring now to FIG. 1, a first substrate including a trench is shownaccording to one aspect of the present embodiments. A first substrate102 is provided. The first substrate 102 may be, for example, a siliconwafer, however it is understood that embodiments are not limited tosilicon wafers. The first substrate 102 includes a first side 104 and asecond side 106. A trench 108 is formed in the first side 104 of thefirst substrate 102 by any suitable fabrication method (e.g. etching,cutting, laser ablation, etc.). The trench 108 may also be referred toas a chimney, and in various embodiments the trench 108 is a high aspectratio trench (e.g. <70:1). In some embodiments the width of the trench108 is 2 to 4 microns, and the length of the trench 108 is not limited.In various embodiments at this stage of fabrication, the trench 108 doesnot extend from the first side 104 entirely to the second side 106 (e.g.the trench 108 stops before reaching the second side 106).

Referring now to FIG. 2, the substrate with the addition of cavities isshown according to one aspect of the present embodiments. A first cavity210 and a second cavity 212 (also referred to as upper cavities) areformed in the first side 104 of the first substrate 102 by any suitablefabrication method (e.g. etching, cutting, laser ablation, etc.). Thefirst cavity 210 is connected to the trench 108, and the trench 108 ispositioned such that it is not over the moveable portion of the MEMSstructure in order to minimize process impact on the moveable structure.The second cavity 212 does not include a trench structure.

It is understood that for simplicity of illustration only two cavitiesare shown, but any number of cavities and trenches may be fabricated ona substrate. In addition, the patterns on the substrate are not limited.For example, cavities without trenches may be next to each other, andcavities with trenches may be next to each other.

In various embodiments, the trench 108 and the first cavity 210 share acommon and fluidly connected open space, such that the trench 108 opensdirectly down into the first cavity 210. In the illustrated embodiment,the trench 108 shares a left side wall with the first cavity 210.However, it is understood that the trench 108 may be on either side ofthe first cavity 210. Furthermore in some embodiments, the trench 108may be positioned in any offset location above the first cavity 210 thatis not directly above a MEMS structure (not shown, but see FIG. 5).

Referring now to FIG. 3, a thermal oxide growth on the trench and thecavities is shown according to one aspect of the present embodiments. Alining 314 is formed over the first side 104 using any suitablefabrication method (e.g. deposition, growth, sputtering, etc.). Forexample, the lining may be a thermal oxide growth (e.g. silicondioxide). The lining 314 covers the first side 104, the trench 108, thefirst cavity 210, and the second cavity 212. In various embodiments, thelining 314 may be used to set or reduce the width of the trench 108 to apredetermined width. For example, the width of the trench 108 after theformation of the lining may be 0.5 to 2.0 microns. In some embodiments,the narrowing of the width of the trench 108 improves the sealing of thetrench (not shown, but see FIG. 7).

Referring now to FIG. 4, the first substrate fusion bonded to a secondsubstrate including standoffs is shown according to one aspect of thepresent embodiments. A second substrate 416 is bonded to the firstsubstrate 102. For example, the second substrate 416 may be fusionbonded to the first substrate 102, thereby forming a silicon oxidesilicon bond. In various embodiments, the second substrate 416 is astructural layer including silicon with MEMS features (e.g. bump stop,damping hole, via, port, plate, proof mass, standoff, spring, seal ring,proof mass, etc.). The MEMS features (not shown, but see FIG. 5) mayinclude one or more moveable elements.

In various embodiments, the first substrate 102 is a MEMS substrate. TheMEMS substrate (first substrate 102) provides mechanical support for thestructural layer (second substrate 416). The MEMS substrate may also bereferred to as a handle substrate or handle wafer. In some embodiments,the handle substrate serves as a cap to the structural layer (e.g.forming the first cavity 210 and the second cavity 212), and may bereferred to as a cap layer.

In some embodiments, standoffs 418 are patterned on the second substrate416. The standoffs 418 define the vertical clearance between thestructural layer and an IC substrate (not shown, but see FIG. 5). Thestandoffs 418 may also provide electrical contact between the structurallayer and the IC substrate.

Referring now to FIG. 5, the formation of MEMS features and eutecticbonding of the second substrate to a third substrate is shown accordingto one aspect of the present embodiments. First MEMS features 520 andsecond MEMS features 521 have been formed in the second substrate 416using any suitable fabrication method. Fabrication methods may include,but are not limited to, etching, cutting, laser ablation, deposition,growth, sputtering, etc.

In addition, the second substrate 416 has been bonded to an additionalsubstrate (e.g. a third substrate 522). For example, in some embodimentsthe standoffs 418 form a eutectic bond 524 with bond pads 526 on thethird substrate 522, thereby providing an electrical connection betweenthe second and third substrates. In various embodiments, the eutecticbond 524 is an aluminum-germanium (AlGe) bond as described in U.S. Pat.No. 7,442,570, the contents of which are incorporated by reference. Inother embodiments, the eutectic bond can be formed by tin-copper,tin-aluminum, gold-germanium, gold-tin, or gold-indium. In someembodiments, the third substrate 522 is an integrated circuit (“IC”)substrate. An IC substrate may refer to a silicon substrate withelectrical circuits, for example CMOS (“complementary metal-oxidesemiconductor”) electrical circuits.

In some embodiments, the standoffs 418 and the third substrate 522define a first lower cavity 528 and a second lower cavity 529. Thestandoffs 418 and the eutectic bond 524 provide a seal that defines afirst enclosure 530 and a second enclosure 531. In various embodiments,the enclosure may refer to a fully enclosed volume typically surroundingthe MEMS structure and typically formed by the IC substrate, structurallayer, MEMS substrate, and the standoff seal ring. For example in theillustrated embodiment, the first enclosure 530 refers to the fullyenclosed volume surrounding the first MEMS features 520, including thefirst cavity 210 and the first lower cavity 528. In addition, the secondenclosure 531 refers to the fully enclosed volume surrounding the secondMEMS features 521, including the second cavity 212 and the second lowercavity 529. In further embodiments, the eutectic bond 524 may besubstituted with other forms of bonding (e.g. solder, adhesive, etc.).In some embodiments, the seal may be a hermetic seal.

Before bonding the second substrate 416 and the third substrate 522,thereby sealing the first enclosure 530 and the second enclosure 531,the pressure and/or environment may be set to a predetermined state. Forexample, the chamber in which the bonding is being performed may beadjusted to increase or decrease the ambient pressure, thereby causingthe pressure in the first enclosure 530 and the second enclosure 531 tomatch the altered ambient pressure. Likewise, the chamber in which thebonding is being performed may be adjusted to increase or decreaselevels of gas (e.g. helium, hydrogen, nitrogen, etc.), thereby causingthe gas levels in the first enclosure 530 and the second enclosure 531to match the altered ambient gas composition. After the gas has beenadjusted, the first enclosure 530 and the second enclosure 531 aresealed, thus trapping the gas pressure and composition within.

Referring now to FIG. 6, removal of a portion of the first substrate toreveal the trench and adjust the pressure in one cavity is shownaccording to one aspect of the present embodiments. A portion of thesecond side 106 of the first substrate 102 is removed to open the trench108 and expose a portion 634 of the lining 314. Removal of the portionof the second side may be done through any suitable method (e.g.etching, cutting, polishing, grinding, etc.). It is understood that theremoval of the portion of the second side 106 is controlled to exposethe portion 634 of the lining 314 in the trench 108, while not exposingthe lining 314 in the second enclosure 531. Thus, the second enclosure531 remains sealed.

In the present embodiment, the portion 634 of the lining 314 ispermeable to various gasses. In some embodiments, the portion 634 of thelining 314 is permeable to all gasses. In further embodiments, theportion 634 of the lining 314 is permeable to only one or only a fewgasses (e.g. helium and/or hydrogen).

As a result of the permeability of the portion 634 of the lining 314,the pressure and/or environment within the first enclosure 530 may beadjusted to a second predetermined state. For example, the chamber inwhich the post processing is being performed may be adjusted to increaseor decrease the ambient pressure, thereby causing the pressure in thefirst enclosure 530 to match the altered ambient pressure. Likewise, thechamber in which the post processing is being performed may be adjustedto increase or decrease levels of gas (e.g. helium, hydrogen, etc.),thereby causing the gas levels in the first enclosure 530 to match thealtered ambient gas composition.

While the environment within the first enclosure 530 is being adjusted,the second enclosure 531 remains sealed. As a result, different MEMSdevices with different internal environments may be created on the sameMEMS wafer. Some of the advantages include reducing process steps,speeding up fabrication, and allowing different MEMS devices to be moreclosely integrated. For example, an accelerometer may optimally functionin a higher pressure environment. In addition, a gyroscope may optimallyfunction in lower pressure environments. Embodiments allow fabricationof both the accelerometer and the gyroscope using shared process steps.

Referring now to FIG. 7, the addition of a seal to set the adjustedpressure of the trench cavity is shown according to one aspect of thepresent embodiments. A seal layer 736 is formed over the second side 106of the first substrate 102 using any suitable fabrication method (e.g.deposition, growth, sputtering, etc.). The seal layer 736 covers theportion 634 of the lining 314, thereby sealing the trench 108 and thefirst enclosure 530. For example, a metal layer (e.g. Al or AlCu) may bedeposited by physical vapor deposition on the second side 106 of thesubstrate 102. The metal layer traps the gas within the trench 108 andthe first enclosure 530, thereby locking in the environment surroundingthe corresponding MEMS device. In various embodiments, the seal layer736 hermetically seals the trench 108 and the first enclosure 530.

In the illustrated embodiment, the seal layer 736 covers the entiresecond side 106 of the first substrate 102. However in some embodiments,the seal layer 736 may cover only portions of the second side 106. Forexample, the seal layer 736 may only cover the portion 634 of the lining314 and an area of the second side 106 surrounding the seal layer 736.Thus the seal layer 736 may be selectively applied in discrete areas toseal the trench 108 and the first enclosure 530.

As previously stated, the arrangement of enclosures with and withouttrenches is not limited. As a result, a number of combinations of MEMSdevices may be fabricated using the described embodiments. For example,MEMS devices requiring a first environment may be fabricated directlynext to each other, or they may be separated by one or more MEMS devicesrequiring a second environment. Thus, many different combinations ofMEMS devices may be fabricated, and should not be limited by theembodiments described herein.

Referring now to FIG. 8, a first substrate including a trench is shownaccording to one aspect of the present embodiments. A first substrate802 is provided. The first substrate 802 may be, for example, a siliconwafer, however it is understood that embodiments are not limited tosilicon wafers. The first substrate 802 includes a first side 804 and asecond side 806. A trench 808 is formed in the first side 804 of thefirst substrate 802 by any suitable fabrication method (e.g. etching,cutting, laser ablation, etc.). The trench 808 may also be referred toas a chimney, and in various embodiments the trench 808 is a high aspectratio trench (e.g. <70:1). In some embodiments the width of the trench808 is 2 to 4 microns, and the length of the trench 808 is not limited.In various embodiments at this stage of fabrication, the trench 808 doesnot extend from the first side 804 entirely to the second side 806 (e.g.the trench 808 stops before reaching the second side 806).

Referring now to FIG. 9, the substrate with the addition of cavities isshown according to one aspect of the present embodiments. A first cavity910 and a second cavity 912 (also referred to as upper cavities) areformed in the first side 804 of the first substrate 802 by any suitablefabrication method (e.g. etching, cutting, laser ablation, etc.). Thefirst cavity 910 is connected to the trench 808 and the first cavity910. It is understood that it can also be stated that the trench 808 islocated within the first cavity 910. As such, the trench 808 ispositioned such that it is not over the moveable structure of the firstcavity 910. The second cavity 912 does not include a trench structure.

It is understood that for simplicity of illustration only two cavitiesare shown, but any number of cavities and trenches may be fabricated ona substrate. In addition, the patterns on the substrate are not limited.For example, cavities without trenches may be next to each other, andcavities with trenches may be next to each other.

In various embodiments, the trench 808 and the first cavity 910 share acommon and fluidly connected open space, such that the trench 808 opensdirectly down into the first cavity 910. In the illustrated embodiment,the trench 808 shares a left side wall with the first cavity 910.However, it is understood that the trench 808 may be on either side ofthe first cavity 910. Furthermore in some embodiments, the trench 808may be positioned in any offset location above the first cavity 910 thatis not directly above a MEMS structure (not shown, but see FIG. 5).

Referring now to FIG. 10, a thermal oxide growth on the trench and thecavities is shown according to one aspect of the present embodiments. Alining 1014 is formed over the first side 804 using any suitablefabrication method (e.g. deposition, growth, sputtering, etc.). Forexample, the lining may be a thermal oxide growth (e.g. silicondioxide). The lining 1014 covers the first side 804, the trench 808, thefirst cavity 910, and the second cavity 912. In various embodiments, thelining 1014 may be used to set or reduce the width of the trench 808 toa predetermined width. For example, the width of the trench 808 afterthe formation of the lining may be 0.5 to 2.0 microns. In someembodiments, the narrowing of the width of the trench 808 improves thesealing of the trench (not shown, but see FIG. 15).

Referring now to FIG. 11, the first substrate fusion bonded to a secondsubstrate including standoffs is shown according to one aspect of thepresent embodiments. A second substrate 1116 is bonded to the firstsubstrate 802. For example, the second substrate 1116 may be fusionbonded to the first substrate 802, thereby forming a silicon oxidesilicon bond. In various embodiments, the second substrate 1116 is astructural layer including silicon with MEMS features (e.g. bump stop,damping hole, via, port, plate, proof mass, standoff, spring, seal ring,proof mass, etc.). The MEMS features (not shown, but see FIG. 12) mayinclude one or more moveable elements.

In various embodiments, the first substrate 802 is a MEMS substrate. TheMEMS substrate (first substrate 802) provides mechanical support for thestructural layer (second substrate 1116). The MEMS substrate may also bereferred to as a handle substrate or handle wafer. In some embodiments,the handle substrate serves as a cap to the structural layer (e.g.forming the first cavity 910 and the second cavity 912), and may bereferred to as a cap layer.

In some embodiments, standoffs 1118 are patterned on the secondsubstrate 1116. The standoffs 1118 define the vertical clearance betweenthe structural layer and an IC substrate (not shown, but see FIG. 12).The standoffs 1118 may also provide electrical contact between thestructural layer and the IC substrate.

Referring now to FIG. 12, the formation of MEMS features and eutecticbonding of the second substrate to a third substrate is shown accordingto one aspect of the present embodiments. First MEMS features 1220 andsecond MEMS features 1221 have been formed in the second substrate 1116using any suitable fabrication method. Fabrication methods may include,but are not limited to, etching, cutting, laser ablation, deposition,growth, sputtering, etc.

In addition, the second substrate 1116 has been bonded to a thirdsubstrate 1222 (e.g. an additional substrate). For example, in someembodiments the standoffs 1118 form a eutectic bond 1224 with bond pads1226 on the third substrate 1222, thereby providing an electricalconnection between the second and third substrates. In variousembodiments, the eutectic bond 1224 is an aluminum-germanium (AlGe)bond. In some embodiments, the third substrate 1222 is an integratedcircuit (“IC”) substrate. An IC substrate may refer to a siliconsubstrate with electrical circuits, for example CMOS (“complementarymetal-oxide semiconductor”) electrical circuits.

In some embodiments, the standoffs 1118 and the third substrate 1222define a first lower cavity 1228 and a second lower cavity 1229. Thestandoffs 1118 and the eutectic bond 1224 provide a seal that defines afirst enclosure 1230 and a second enclosure 1231. In variousembodiments, the enclosure may refer to a fully enclosed volumetypically surrounding the MEMS structure and typically formed by the ICsubstrate, structural layer, MEMS substrate, and the standoff seal ring.For example in the illustrated embodiment, the first enclosure 1230refers to the fully enclosed volume surrounding the first MEMS features1220, including the first cavity 910 and the first lower cavity 1228. Inaddition, the second enclosure 1231 refers to the fully enclosed volumesurrounding the second MEMS features 1221, including the second cavity912 and the second lower cavity 1229. In further embodiments, theeutectic bond 1224 may be substituted with other forms of bonding (e.g.solder, adhesive, etc.). In some embodiments, the seal may be a hermeticseal.

Before bonding the second substrate 1116 and the third substrate 1222,thereby sealing the first enclosure 1230 and the second enclosure 1231,the pressure and/or environment may be set to a predetermined state. Forexample, the chamber in which the bonding is being performed may beadjusted to increase or decrease the ambient pressure, thereby causingthe pressure in the first enclosure 1230 and the second enclosure 1231to match the altered ambient pressure. Likewise, the chamber in whichthe bonding is being performed may be adjusted to increase or decreaselevels of gas (e.g. helium, hydrogen, nitrogen, etc.), thereby causingthe gas levels in the first enclosure 1230 and the second enclosure 1231to match the altered ambient gas composition. After the gas has beenadjusted, the first enclosure 1230 and the second enclosure 1231 aresealed, thus trapping the gas pressure and composition within.

Referring now to FIG. 13, removal of a portion of the first substrate toreveal the trench is shown according to one aspect of the presentembodiments. A portion of the second side 806 of the first substrate 802is removed to open the trench 808 and expose a portion 1334 of thelining 1014. Removal of the portion of the second side may be donethrough any suitable method (e.g. etching, cutting, polishing, grinding,etc.). It is understood that the removal of the portion of the secondside 806 is controlled to expose the portion 1334 of the lining 1014 inthe trench 808, while not exposing the lining 1014 in the secondenclosure 1231. Thus, the second enclosure 1231 remains sealed.

Referring now to FIG. 14, removal of the exposed portion of the liningto reveal the trench and adjust the pressure in one cavity is shownaccording to one aspect of the present embodiments. The exposed portion1334 of the lining 1014 (e.g oxide layer) is removed by any suitablemethod (e.g. etching, cutting, polishing, grinding, etc.), therebycreating an opening 1435 within the trench 808. In some embodiments,only a portion of the exposed portion 1334 of the lining 1014 isremoved. For example, an oxide layer may line the trench, therebyforming a trench lining. An portion of the oxide layer may be exposed,thereby forming an exposed oxide portion of the trench lining. Theentire exposed oxide portion of the trench lining or only a portion ofthe exposed oxide portion of the trench lining may then be removed.

As a result of the opening 1435, the pressure and/or environment withinthe first enclosure 1230 may be adjusted to a second predeterminedstate. For example, the chamber in which the post processing is beingperformed may be adjusted to increase or decrease the ambient pressure,thereby causing the pressure in the first enclosure 1230 to match thealtered ambient pressure. Likewise, the chamber in which the postprocessing is being performed may be adjusted to increase or decreaselevels of gas (e.g. helium, hydrogen, etc.), thereby causing the gaslevels in the first enclosure 1230 to match the altered ambient gascomposition.

While the environment within the first enclosure 1230 is being adjusted,the second enclosure 1231 remains sealed. As a result, different MEMSdevices with different internal environments may be created on the sameMEMS wafer. Some of the advantages include reducing process steps,speeding up fabrication, and allowing different MEMS devices to be moreclosely integrated. For example, an accelerometer may optimally functionin a higher pressure environment (e.g. greater than 50 millibar). Inaddition, a gyroscope may optimally function in lower pressureenvironments (e.g. less than 8 millibar). Embodiments allow fabricationof both the accelerometer and the gyroscope using shared process steps.

Referring now to FIG. 15, the addition of seals to set the adjustedpressure of the trench cavity is shown according to one aspect of thepresent embodiments. A first seal layer 1536 (i.e. oxide layer) and asecond seal layer 1538 are formed over the second side 806 of the firstsubstrate 802 using any suitable fabrication method (e.g. deposition,growth, sputtering, etc.). The first seal layer 1536 and the second seallayer 1538 cover the opening 1435, thereby sealing the trench 808 andthe first enclosure 1230. In various embodiments, the first seal layer1536 and the second seal layer 1538 hermetically seal the trench 808 andthe first enclosure 1230.

For example, an oxide seal may be deposited by chemical vapor depositionon the second side 806 of the substrate 802. In addition, a metal layer(e.g. AlCu) may be deposited by physical vapor deposition on the oxideseal. The metal and oxide layers trap the gas within the trench 808 andthe first enclosure 1230, thereby locking in the environment surroundingthe corresponding MEMS device. It is understood that the metal layer andoxide layer are exemplary and not limiting. In other embodiments, theoxide layer may overlay the metal layer. In further embodiments, sealsother than metal and oxides may be used. In some embodiments, one layeror more than two layers may be used as seals.

In the illustrated embodiment, the first seal layer 1536 and the secondseal layer 1538 cover the entire second side 806 of the first substrate802. However in some embodiments, the first seal layer 1536 and thesecond seal layer 1538 may cover only portions of the second side 806.For example, the first seal layer 1536 and the second seal layer 1538may only cover the opening 1435 and an area of the second side 806surrounding the opening 1435. Thus the first seal layer 1536 and thesecond seal layer 1538 may be selectively applied in discrete areas toseal the trench 808 and the first enclosure 1230.

As previously stated, the arrangement of enclosures with and withouttrenches is not limited. As a result, a number of combinations of MEMSdevices may be fabricated using the described embodiments. For example,MEMS devices requiring a first environment may be fabricated directlynext to each other, or they may be separated by one or more MEMS devicesrequiring a second environment. Thus, many different combinations ofMEMS devices may be fabricated, and should not be limited by theembodiments described herein.

In embodiments where the metal layer is in direct contact with thesecond side 806, the metal layer advantageously provides an EMC shield.In addition, in embodiments including the metal layer, the seal qualityis advantageously improved.

FIG. 16 an exemplary flow diagram for setting a pressure within a trenchand cavity is shown according to one aspect of the present embodiments.At a block 1650, a trench is formed in a first side of a first siliconwafer. For example, in FIG. 1 a trench is formed in the first side ofthe first substrate by any suitable fabrication method (e.g. etching,cutting, laser ablation, etc.).

At a block 1652, a cavity connected to the trench is formed in the firstside of the first silicon wafer. For example, in FIG. 2 a first cavityis formed in the first side of the first substrate by any suitablefabrication method (e.g. etching, cutting, laser ablation, etc.). Thefirst cavity is connected to the trench.

In some embodiments, an additional cavity is formed in the first side ofthe silicon wafer. For example, in FIG. 2 a second cavity 212 is formedin the first side of the first substrate by any suitable fabricationmethod (e.g. etching, cutting, laser ablation, etc.). In someembodiments, the additional cavity in the first side of the firstsilicon wafer includes a different pressure from the pressure within thetrench and the cavity. For example, in FIG. 6 while the environmentwithin the first enclosure is being adjusted, the second enclosureremains sealed. As a result, different MEMS devices with differentinternal environments may be created on the same MEMS wafer.

At a block 1654, an oxide layer is formed on the first side and in thetrench. For example, in FIG. 3 a lining is formed over the first sideusing any suitable fabrication method (e.g. deposition, growth,sputtering, etc.). In some embodiments, the lining may be a thermaloxide growth (e.g. silicon dioxide) that reduces the width of thetrench. For example, in FIG. 3 the lining may be used to set or reducethe width of the trench to 0.5 to 2.0 microns.

At a block 1656, the first side of the first silicon wafer is bonded toa second silicon wafer. For example, in FIG. 4 a second substrate isbonded to the first substrate.

At a block 1658, a MEMS structure is formed in the second silicon wafer.For example, in FIG. 5 first MEMS features are formed in the secondsubstrate using any suitable fabrication method. Fabrication methods mayinclude, but are not limited to, etching, cutting, laser ablation,deposition, growth, sputtering, etc. In some embodiments, an additionalMEMS structure is formed in the second silicon wafer. For example, inFIG. 5 second MEMS features have been formed in the second substrateusing any suitable fabrication method.

In some embodiments, a standoff is formed on the second silicon wafer.For example, in FIG. 4 standoffs are patterned on the second substrate.The standoffs define the vertical clearance between the structural layerand an IC substrate, as well as providing electrical contact between thestructural layer and the IC substrate.

At a block 1660, the second silicon wafer is bonded to a third siliconwafer, wherein the bonding seals the MEMS structure between the thirdsilicon wafer and the cavity. For example, in FIG. 5 the secondsubstrate has been bonded to a third substrate with a eutectic bond,thereby sealing a first enclosure. The first enclosure is the fullyenclosed volume surrounding the first MEMS features, including the firstcavity and the first lower cavity.

In some embodiments, the bonding the second silicon wafer to the thirdsilicon wafer provides a hermetic seal and an electrical connection. Forexample, in FIG. 5 the eutectic bond is a seal that may be a hermeticseal. In addition, the eutectic bond provides an electrical connectionbetween the second substrate and the third substrate. In furtherembodiments, the bonding the second silicon wafer to the third siliconwafer includes forming an aluminum-germanium bond. For example, in FIG.5 the eutectic bond may be an aluminum-germanium (AlGe) bond.

At a block 1662, a portion of a second side of the first silicon waferis removed, wherein the removing exposes the trench. For example, inFIG. 6 a portion of the second side of the first substrate is removed toopen the trench.

In some embodiments, a lining is formed within the trench, which reducesthe width of the trench. For example, in FIG. 3 a lining is formed overthe first side using any suitable fabrication method (e.g. deposition,growth, sputtering, etc.), and the formation of the lining reduces thewidth of the trench. In further embodiments, the removal of the portionof the second side of the first silicon wafer exposes a portion of thelining, and the exposed portion of the lining is removed. For example,in FIG. 14 the exposed portion of the lining is removed by any suitablemethod (e.g. etching, cutting, polishing, grinding, etc.), therebycreating an opening within the trench.

At a block 1664, a pressure is set within the trench and the cavity. Forexample, in FIG. 6 the pressure and/or environment within the firstenclosure is adjusted to a second predetermined state. In someembodiments, the oxide layer is permeable to a gas, and setting thepressure within the trench and the cavity includes passing the gasthrough the oxide layer. For example, in FIG. 6 the portion of thelining is permeable to various gasses. As a result of the permeabilityof the portion of the lining, the pressure and/or environment within thefirst enclosure may be adjusted to a second predetermined state.

At a block 1666, a layer is formed on the second side of the firstsilicon wafer, wherein the layer seals the trench, the cavity, and theMEMS structure. For example, in FIG. 7 a seal layer is formed over thesecond side of the first substrate using any suitable fabrication method(e.g. deposition, growth, sputtering, etc.).

FIG. 17 an exemplary flow diagram for sealing a first pressure in asecond cavity and sealing a second pressure in a first cavity is shownaccording to one aspect of the present embodiments. At a block 1770, atrench is formed in a handle substrate. For example, in FIG. 8 a trenchis formed in the first side of the first substrate by any suitablefabrication method (e.g. etching, cutting, laser ablation, etc.). Thefirst substrate provides mechanical support for the structural layer andmay be referred to as a handle substrate or handle wafer. In someembodiments, the handle substrate serves as a cap to the structurallayer.

At a block 1772, a trench lining is formed in the trench. For example,in FIG. 10 a lining is formed over the first side and may be a thermaloxide growth (e.g. silicon dioxide). The lining covers the first side,the trench, the first cavity, and the second cavity. In someembodiments, the width of the trench is 2 to 4 microns before theforming the trench lining, and the width of the trench is 0.5 to 2.0microns after the forming the trench lining. For example, in FIG. 8 thewidth of the trench may be 2 to 4 microns, and in FIG. 10 the width ofthe trench after the formation of the lining may be 0.5 to 2.0 microns.

At a block 1774, a first cavity and a second cavity are formed in thehandle substrate, wherein the first cavity is connected to the trench.For example, in FIG. 9 a first cavity and a second cavity (also referredto as upper cavities) are formed in the first side of the firstsubstrate (also referred to as a handle substrate) by any suitablefabrication method (e.g. etching, cutting, laser ablation, etc.). Thefirst cavity is connected to the trench.

At a block 1776, a first MEMS structure and the handle substrate aresealed for maintaining a first pressure within the trench and the firstcavity. For example, in FIG. 12 MEMS features have been formed in thesecond substrate using any suitable fabrication method. In addition, thebonding of the second substrate to the third substrate seals apredetermined pressure and/or environment within the trench and firstcavity. The MEMS structure is within the first cavity, and the firstcavity includes a side opposite the MEMS structure. The trench isconnected to the side of the cavity opposite the MEMS structure, and thetrench is not directly opposite the MEMS structure.

At a block 1778, a second MEMS structure and the handle substrate aresealed for maintaining the first pressure within the second cavity. Forexample, in FIG. 12 the bonding of the second substrate to the thirdsubstrate seals a predetermined pressure and/or environment within thesecond cavity containing the MEMS features.

At a block 1780, a portion of the trench lining is exposed. For example,in FIG. 13 a portion of the second side of the first substrate isremoved to open the trench and expose a portion of the lining. In someembodiments, the exposed portion of the trench lining or a portion ofthe exposed portion of the trench lining is removed. For example, inFIG. 14 the exposed portion of the lining is removed by any suitablemethod (e.g. etching, cutting, polishing, grinding, etc.), therebycreating an opening within the trench.

In other embodiments, the exposed portion of the lining is not removed.Instead the trench lining is permeable to a gas, and changing the firstpressure to a second pressure within the first cavity includes passingthe gas through the trench lining. For example, in FIG. 6 the portion ofthe lining is permeable to one or more gasses (e.g. helium and/orhydrogen). As a result of the permeability of the portion of the lining,the pressure and/or environment within the first enclosure may beadjusted to a second predetermined state.

At a block 1782, the first pressure is changed to a second pressurewithin the first cavity. For example, in FIG. 14 the pressure and/orenvironment within the first enclosure may be adjusted to a secondpredetermined state.

At a block 1784, the first cavity and the trench are sealed to maintainthe second pressure within the trench and the first cavity. For example,in FIG. 15 a first seal layer and a second seal layer are formed overthe second side of the first substrate using any suitable fabricationmethod (e.g. deposition, growth, sputtering, etc.). In some embodiments,the sealing includes depositing a metal layer over the trench lining.For example, in FIG. 15, a metal layer (e.g. AL or AlCu) may bedeposited by physical vapor deposition on the oxide seal. The metal andoxide layers trap the gas within the trench and the first enclosure,thereby locking in the environment surrounding the corresponding MEMSdevice.

While the embodiments have been described and/or illustrated by means ofparticular examples, and while these embodiments and/or examples havebeen described in considerable detail, it is not the intention of theApplicants to restrict or in any way limit the scope of the embodimentsto such detail. Additional adaptations and/or modifications of theembodiments may readily appear to persons having ordinary skill in theart to which the embodiments pertain, and, in its broader aspects, theembodiments may encompass these adaptations and/or modifications.Accordingly, departures may be made from the foregoing embodimentsand/or examples without departing from the scope of the conceptsdescribed herein. The implementations described above and otherimplementations are within the scope of the following claims.

What is claimed is:
 1. An apparatus comprising: a cavity within asubstrate; a MEMS structure surrounded by the cavity; a trench connectedto the cavity, wherein the trench is not directly opposite the MEMSstructure; an oxide layer lining the trench and the cavity; and a seallayer, wherein the seal layer seals the trench and traps a predeterminedpressure within the cavity and the trench.
 2. The apparatus of claim 1,further comprising: an additional cavity including an additionalpredetermined pressure, wherein the additional predetermined pressure isdifferent from the predetermined pressure; and an additional MEMSstructure surrounded by the additional cavity.
 3. The apparatus of claim1, wherein the additional cavity does not include an additional trench.4. The apparatus of claim 1, wherein the substrate is a handlesubstrate; and further comprising: a structural layer including the MEMSstructure; and a fusion bond connecting the handle substrate to thestructural layer.
 5. The apparatus of claim 4, wherein the structurallayer is eutecticly bonded to an additional substrate.
 6. The apparatusof claim 5, wherein the additional substrate has CMOS (complementarymetal-oxide semiconductor) electrical circuits.
 7. An apparatuscomprising: a trench in a first side of a first silicon wafer; a cavityconnected to the trench in the first side of the first silicon wafer; anoxide layer on the first side and in the trench; a second silicon waferbonded to the first side of the first silicon wafer; a MEMS(micro-electro-mechanical systems) structure in the second siliconwafer; a third silicon wafer bonded to the second silicon wafer, a layeron a second side of the first silicon wafer, wherein the trench isexposed in a portion of the second side of the first silicon wafer, thelayer seals the trench, the cavity, and the MEMS structure, and thelayer seals a pressure in the cavity.
 8. The apparatus of claim 7,wherein the second silicon wafer is eutecticly bonded to the thirdsilicon wafer.
 9. The apparatus of claim 7, further comprising anadditional cavity in the first side of the first silicon wafer.
 10. Theapparatus of claim 9, wherein the additional cavity in the first side ofthe first silicon wafer includes a different pressure from the pressurewithin the cavity.
 11. The apparatus of claim 7, further comprising anadditional MEMS structure in the second silicon wafer.
 12. The apparatusof claim 7, wherein the second silicon wafer bonded to the third siliconwafer provides a hermetic seal and an electrical connection.
 13. Theapparatus of claim 7, wherein the second silicon wafer bonded to thethird silicon wafer includes an aluminum-germanium bond.
 14. Theapparatus of claim 7, wherein the oxide layer reduces a width of thetrench.